This invention generally relates to radioactive waste disposal technology. In particular, this invention relates to radioactive wet wastes solidification. More particularly still, this invention relates to co-solidification of low-level radioactive wet wastes which are produced from BWR nuclear power plants.
In operation, the boiling water reactor (BWR) used in a BWR nuclear power plant produces wet wastes such as liquid sodium sulfate concentrate waste, powdery spent ion-exchange resin and sludge wastes. These wet wastes are radioactive and therefore must need be subject to solidification treatment and made into chemically as well as physically stable solid matters before their final disposal as the common safety measures for radioactive wastes require.
At present, there are three kinds of solidification treatments for low-level radioactive wet wastes: they are cement solidification, polymer solidification and bitumen solidification. Among these solidification methods, cement solidification presents the worst volume efficiency; accordingly, even though its operation is the simplest of the three and its product, the cement-solidified waste, has the required long-term stability, it is scarcely considered as an attractive method because of the high cost that reflects in the final disposal procedures.
Polymer solidification and bitumen solidification employ organic materials for their solidification agents, and both are of high volume efficiency. However, with regard to bitumen solidification, bitumen-solidified waste is flammable besides its low compressive strength. Once in Germany such solidified waste burst into flames during a bitumen solidification process; and, some years ago, one of the Japanese bitumen solidification systems exploded and brought about a serious radioactive accident to the worry of the whole world. Many countries in Europe have since prohibited the operation of bitumen solidification; and, in the rest of the world, bitumen solidification systems and plants are being closed down one by one.
As to the use of polymer solidification, this is a highly controversial issue; while, in spite of that, new polymer solidification systems still keep coming out. Those who are against it argue that, the stability of polymer-solidified waste may be dangerously unreliable because of polymer aging. Although many countries no longer approve polymer solidification in the treatment of radioactive wet wastes, this method is still widely used in some other countries for the advantage of its high volume efficiency.
Under the circumstances, the principal research direction for low-level radioactive wet wastes solidification is to increase the volume efficiency of inorganic solidification agents, in the hope that by which the organic method may be replaced as soon as possible.
The traditional cement solidification is just such an inorganic method. One problem this method often meets with is that, in the process of solidification of sulfate liquid waste, sulfate reacts with tricalcium aluminate, 3CaO.Al2O3, forming gradually a low-density solid matter called ettringite, which as a rule causes distortion and sometimes even cracks in the solidified waste owing to volume expansion. To this problem two obvious preventive measures are (1) to decrease the sulfate to cement ratio and (2) to reduce tricalcium aluminate content in cement. The former is not at all interesting, since it results in much larger solidified waste and consequently much greater cost in the final disposal procedures. While the latter is far from satisfactory, not only because the cement with low tricalcium aluminate content is not easily available, but mainly because the formation of ettringite is so slow a process that long-term stability of such solidified waste is extremely doubtful.
In U.S. Pat. No. 04,804,498 a strategy is proposed to get rid of the aforesaid problem caused by sodium sulfate which is highly reactive and easily soluble. It is to have sodium sulfate reacted with barium hydroxide to become barium sulfate and sodium hydroxide, and then separate the two, and have barium sulfate solidified and sodium hydroxide recycled for reuse. Thanks to the high stability and the extremely low solubility of barium sulfate, the solidified waste so produced is very stable, free from the troubles often encountered in the solidification of sodium sulfate liquid waste. Nevertheless, with one problem solved a new one is created immediately. For the reason that the separated sodium hydroxide takes with it most of the radioactive elements, further decontamination procedures are needed before it is able to be recycled for reuse; and usually the recycled chemical soon loses its potency after a few runs of recycling because of the speedy build-up of its content of contaminants; therefore, in the end, solidification treatment still has to be resorted to.
A Japan Laid-Open Patent Publication (No. 62, 126, 400) reports a solidification method relevant to the present disclosure, in which sodium sulfate liquid waste is dried into powder, then it is mixed with barium hydroxide, resulting in water, sodium hydroxide and the insoluble barium sulfate; and then silicon dioxide and solidification agent are introduced to facilitate solidification. High energy cost in the use of vaporization dryer is a major drawback of this method, besides a few engineering problems such as solid-solid reaction, agitation, and heat transfer need to be overcome.
Still another Japan Laid-Open Patent Publication (No. 04, 128, 699) discloses a solidification method, in which barium sulfate and sodium hydroxide liquid mixture is produced like the foregoing U.S. patent, only this time without their separation; and subsequently the mixture is concentrated via evaporation, and then silicon dioxide and cement are introduced to solidify the wet wastes. It is known that the quality of cement solidified waste in a great measure depends upon the amount of sodium hydroxide present in the waste. With the reaction of sodium hydroxide and silicon dioxide, sodium silicate are produced; and sodium silicate can react with the calcium ions coming out of the hydration of cement, forming a silicon-calcium gelatinous hydrated product. Obviously, therefore, the quality of solidified waste has a good deal to do with the amount of silicon dioxide and the kind and quantity of cement employed. Specifically addressing to this problem, it is proposed in Japan Laid-Open Patent Publication No. 62, 278, 499 that, if radioactive wet waste is to be solidified with the help of sodium silicate, the silicon to sodium ratio should be kept within the range from 0.5 to 1.0. Because it is found that, when the sodium hydroxide content exceeds 8 wt. %, the compressive strength of the solidified waste becomes lower than 50 kg/cm2. This clearly shows that, even after sodium sulfate liquid waste has been converted to barium sulfate and sodium hydroxide, the quality of cement-solidified waste still much depends on the kinds and quantities of solidification agents in use, and, of course, also on solidification conditions.
As to the solidification of powdery spent ion-exchange resin, in most of the BWR nuclear power plants this kind of waste is solidified with cement. Usually within such solidified waste there is 20% of spent ion-exchange resin by weight. Be that as it may, it is possible that the ion-exchange resin content may reach as high as 30% of the total weight of the solidified waste, and such solidified waste still possesses strong enough compressive strength, as demonstrated in Japan Laid-Open Patent Publication No. 62, 238, 499, where spent ion-exchange resin, having been treated with sodium hydroxide, is solidified by the addition of blast furnace slag powder.
Although some of the above-mentioned solidification treatments can produce solidified wastes of sufficient compressive strength, it may be proper to emphasize over here that, every one of those previous arts only deals with one kind of radioactive wet waste with very limited volume efficiency.
Accordingly, the solidification method of the present invention herein disclosed adopts the strategy of making waste solidified with waste, which can direct concentrated sodium sulfate liquid waste and spent ion-exchange resin to be solidified together. The procedures and principles of this solidification method are the following. First, have sodium sulfate liquid waste react with barium hydroxide, so that the liquid waste is converted into a slurry of barium sulfate and sodium hydroxide. Second, add into the slurry spent ionexchange resin which at once reacts with sodium hydroxide, the reaction being able to increase the stability of the waste by reducing the ion-exchange activity of the resin. Third, have the slurry thoroughly mixed with a solidification agent which is composed of cement, fine silica gel particles, pozzolanic materials (such as blast furnace slag powder and fly ash), silicate, phosphate, etc.
This novel method for radioactive wet wastes solidification at once co-solidifies both sodium sulfate liquid waste and spent ion-exchange resin, and demonstrates the following advantages: (1) By having the chemically very unstable sodium sulfate converted into barium sulfate which has very high stability, this not only warrants the stability of the solidified waste but also gains result in waste volume reduction thanks to the high density (4.5) of barium sulfate. (2) During the process of solidification barium sulfate serves as a fine aggregate material which enhances the strength of solidified waste. (3) By having spent ionexchange resin reacted with sodium hydroxide, the ion-exchange activity of the resin is greatly reduced, so that the problem of solidified waste volume expansion is no longer present. (4) All the converted wastes are solidified together, without producing secondary wastes, and without the complication of waste recycle. (5) With suitable preparation of solidification agent, sodium hydroxide can form with the solidification agent into an insoluble solidified matter which encases and solidifies the other wastes. This technique not only reduces the use of solidification agent but also achieves the goal of making waste solidified with waste.